Electric discharge lamp



Oct. 19, 1937. v. J. FRANCIS ET AL ELETRIC DIS CHARGE LAMP Filed March 2, 1937 INVENTORS Victor James Francis John Walter R d BY W AT ORNEY Patented Oct. 19, 1937 PATENT err-le ELECTRIC DISCHARGE LAMP Victor James Francis, North Wembley,'and John Walter Ryde, London, England, assignors to General Electric Company, a corporation of New York Application March 2, 1937, Serial No. 128,687

In Great Britain February 20, 1936 K 4 Claims.

particularly adapted for use as sources in optical projection apparatus. The term mercury does 5 not exclude the presence of substances, additional to mercury, whose spectra may contribute sub- .stantially to the light from the lamp.

In one'well-known kind of high-pressuremercury-vapour lamp, used for street-lighting and similar purposes, the diameter of the envelope enclosing the discharge is some 25 mm., the pressure of the vapour in operation is 1 to 2 atmospheres, and the brightness of the discharge column does not exceed 300 candles per sq. cm.

Here and hereinafter brightnessmeans brightness of the brightest part of the discharge column in a plane perpendicular to its length.

It is desirable that sources for optical projection should be of greater brightness. It is known that one way in which greater brightness can be obtained is to increase the vapour pressure of 'the mercury in operation to 10 atmospheres or more and to decrease the diameter of the envelope. Thus in British patent specification No. 431,923 lamps are described in which the volts per cm.

of the discharge path exceed 150 and may'attain 400, (which correspond respectively to vapour pressures of about 20 and'lOO atm.) and in which the diameter of the envelope does not exceed The primary object of using a narrow envelope isv to promote the stability of the column. For when the pressure of the vapour is high, the column is apt to wander irregularly about its mean position under the intense convection currents; such wandering occurs if the lamp is operated with the line between the electrodes vertical, and also if it is operated with that line horizontal, and if, as is then usual, a magnetic field is ap- 40 plied to prevent the column from bowing upwards and approaching the wall; the prevention of such wandering is very important if the'lamp is to be used as a source in optical projection. A subsidiary advantage is that it makes the column Z narrower and therefore brighter than it would be ample by an external water-jacket. A brightness This invention relates to high-pressure mer- 'cury vapour electric discharge lamps of the type of 18,000 candles per sq. cm. can thus be obtained; but the useful life of the lamp is apt to be limited by an obscuration 'of the envelope due to the intense heating of its innersurface. The object of the invention is to produce lamps with a 5 brightness of the order of 10,000 candles per sq. cm. that are much less subject to this obscuration.

It is to be observed that a narrow envelope is not essential to high brightness. In British pat- 10 ent specification No. 434,919-lamps are described in which the diameter of the spherical envelope may be as great as 30 mm., and in which a stable discharge column of high brightness'is obtained by making'the ratio of the power dissipated in the discharge to the distance between the electrodes greater (preferably much greater) than watt/cm. The said ratio will hereinafter be denoted for brevity by W/l; if the lamp has runningand starting electrodes, "the elec- 20 trodes always means "the rimning electrodes. Lamps in which stability is obtained by making I W /Z very great do not lie within the scope of this here and hereinafter implies that, during the 30 test for instability, measures are taken to make the mean position of the column coincide, exactly or nearly, with the line. joining its ends. either by making that line vertical or by applying a suitable magnetic field. 35

The invention rests upon two facts, of which the first is obvious when it is pointed out. It is that, since a. lamp used for optical projection is required to emit light only within a limited angle, usually less and often much less than 180, 40 the presence of absorbing matter outside that angle is no serious disadvantage. It may prevent the use of a backing reflector; but if by ren- Qdering the discharge column stable, .it enables Y to the absence of such a reflector is easily offset.

. The second fact is a new discovery. It is that,

in order to obtain some at least of the advantages,

ter adjacent to it. If sufficiently powerful forces act on the discharge column perpendicular to its length, driving it towards suitably shaped solid sufliciently powerful.

matter and causing it to lie adjacent to that matter, it is .not necessary that there should'be .there is a solid surface concave downwards above the said line, and if the forces of convection driving the column towards that solid surface are When the column is 'thus forced to lie adjacent to the solid matter above it, its distance from solid matter below it has no effect on its stability. If the solid surface is flat or the forces insufficiently powerful, then, though the solid may limit the bowing upwards of the column, the column will be liable to wander over its surface. Instead of or in addition to convective forces, those arising from a magnetic field may be employed to drive the column towards solid matter on one side of it; again the distance of solid matter in other directions is relatively unimportant.

In virtue of these facts, the solid matter surrounding the line joining the electrodes may be of two different kinds, performing different functions. The function of the matter on the side towards which the column is driven is to be refractory, so as to resist the heat developed in the column adjacent to it; its optical properties are immaterial. The function of the matter on the side away from which the column is driven is to transmitto the exterior the light from the column; transparency is essential, but no more refractoriness is necessarily required than will enable the matter to stand a temperature suflicient to maintain the vapour pressure of the mercury; this temperature must, of course, be

attained by all parts of the envelope.

According to the invention in a high-pressure mercury-vapour electric discharge lamp, adapted to be so operated that in full operation the -pres-' sure of the vapour is so great and the ratio W/l so small that, if the discharge column were sufficiently distant from solid matter, it would be unstable, the line joining the electrodes has 'on one side of it refractory solid matter and on another side of it a transparent portion of the envelope, the distance between the said refractory matter and the said transparent portion being so great that when the discharge column is driven towards the refractory matter by forces perpendicular to its length, so as to lie stably adjacent to it, the column is so distant from the transparent portion that this portion undergoes substantially no obscuration during life owing to the heat developed in the discharge column. In operation a lamp according to the invention is to be associated with means (which may be gravity producing convection) producing such forces driving the discharge column towards the refractory matter.

Some explanation of the terms used in the foregoing statement must now be added.

The term refractory implies that the matter is not damaged by the heat developed in the column lying adjacent to it. Since the. trans-' parency of the refractory matter is unimportant, mere obscuration of an originally transparent substance is not to regarded as damage. The

refractoriness may be inherent in the nature of the matter, so that no cooling of it is necessary portions adjacent the dischargemust be inparent portion from the discharge.

notbe on the same side of the line. forces driving the discharge column are con-- sulated from each other, in order that the discharge shall not be short-circuited. Then the refractory matter need not be, and usually will "not be, part of the envelope enclosing the discharge; it may be a partition in or completely across the envelope. But the refractoriness may be acquired from artificial cooling applied tothe surface remote from the discharge; then the refractory matter will be usually part of the envelope enclosing the discharge. The refractory matter must be more refractory than the said transparent portion; for otherwise no advantage would be obtained from the distinction between them. But if the refractoriness is acquired solely from cooling, its inherent refractoriness need be no greater. In particular both the refractory matter and the transparent portion may be of quartz, so long as the former is cooled.

The transparent portion will usually be vit'-' reous; but quartz is not essential, since the great-' est refractoriness is not required. It must not produce appreciably more unwanted absorption of the light incident onit than the quartz or i Some loss of transparency due to this course is.

likely to occur however great (subject to practical limitations) is the distance of the trans-- It is almost independent of the obscuration to which the in-' vention is directed, which is due to the high temperature reached by matter near the discharge. I

The refractory matter and the transparent portion need not be in opposite sides of the line joining the electrodes; but of course they can- Thus, if the vective and the refractory matter has to be above the electrodes, the lamp cannot be adapt-1 ed to emit light upwards; on the other hand, it

'can be adapted to emit light horizontally or near- 1y horizontally as well as downwards.

It is preferable that the surface of the refractory matter towards which the discharge column is driven should be highly curved, so that, when the column is lying adjacent to this surface, it is more closely surrounded by matter than it would be if it lay at the same minimum distance from the transparent portion, which will generally be as nearly flat as is consistent with the general shape of the envelope. The stability of the discharge is in general the greater the more closely it-is surrounded by matter. But, if the driving forces are sufficiently strong and sufficiently concentrated in one direction, the discharge may lie stably against a surface whose curvature is no greater, and may possibly be less,- than that of the main part of the envelope.

' Certain embodiments of the invention will now be described with reference to the accompanying drawing, in which Figures 1', 2, 3 each show a different embodiment; Of these Figure 1 illus-: trates the use of an inherently refractory par-i tition, Figure 2 the use of a portion of the envelope cooled by forced air circulation as the refractory matter, Figure 3 the use of an applied water-cooled member to supply the cooling. Each embodiment is to be operated with the top of the figure uppermost; the driving forces may then be purely convective. I

Each figure is a section of the lamp by a plane perpendicular to the line joining the electrodes and midway between them. The point at which this line intersects the plane is indicated by X; but its position may vary somewhat with the length of the said line, which will determine the length of the lamp perpendicular to the plane of the drawing. For the distance between the electrodes will be determined by the voltage of the lamp and the vapour pressure at which it is designed to operate; the distance of X from the solid matter'above it must not be so large compared with the distance between the electrodes that the length of the discharge column is increased very greatly when itis driven to lie adjacent to the refractory solid matter. In all cases, if the lamp is designed for 400 watts, the

' vertical distance between the solid matter above and below X may be about 20 mm.

In Figure 1 the envelope directly enclosing the discharge is a quartz tube l of circular section, whose lower part is the said transparent portion. The refractory solid matter is a V-shaped plate 2' of thoria substantially coextensive in length with the discharge path inserted as a partition in' the tube, with its apex upwards. The

. column is forced to lie near the apex by the con- I vective forces, so that it is closely surrounded by solid matter.

In Figure 2 the envelope directly enclosing the discharge is the lower half 4 of a quartz tube I (which half is the transparent portion) and a quartz partition 5 sealed across the tube, so as to divide it into two channels separate from each other and extending along substantially the entire discharge path. The partition has a "pinched portion 6, into which the column is driven by the convective forces, so that it is closely surrounded by matter. The partition, which is the refractory matter, is cooled by fluid (preferably air) circulated through the channel 1 above the partition.

In Figure 3 the envelope is a plain quartz tube I, to the upper portion of which is applied along.

substantially the entire discharge path a closely fitting metal member 8 furnished with a channel 9 through which water is circulated. This upper portion thus becomes the refractory solid matter. Alternatively the water cooling of the metal member may be replaced by air cooling by means of fins forming part of it.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

1. A high pressure vapor electric discharge lamp comprising a sealed envelope containing a gaseous atmosphere and having a pair of electrodes sealed'therein, the line joining said electrodes having a transparent portion of said envelope on one side thereof and an artificially cooled portion of said envelope on another side thereof, and means to confine the discharge between said electrodes to an eccentric path within said envelope adjacent said artificially cooled portion thereof, whereby the discharge is stabilized and the transparent portion of said wall is maintained unobscured.

2. A high pressure vapor electric discharge lamp comprising a sealed envelope containing a gaseous atmosphere and having a pair of electrodes sealed therein, means to confine the discharge between said electrodes to an eccentric path within said envelope close to one wall thereof to stabilize said discharge and to reduce the heating of the opposite wall of said envelope, and means to force a flow of air over that portion of the envelope to which the discharge is adjacent in order to keep it below the softening temperature.

3. A high pressure vapor electric discharge lamp comprising a sealed envelope containing a gaseous atmosphere and having a pair of electrodes sealed therein, means to confine the discharge between said electrodes to an eccentric path within said envelope close to one wall thereof to stabilize said discharge and to reduce the heating of the opposite wall of said envelope, and means to maintain that portion of the envelope to which the discharge is adjacent below the softening temperature, said means comprising a a metal member in contact with that portion of said envelope and having means associated therewith to provide flow of a cooling fluid to remove heat from said metal members.

4. A high pressure vapor electric discharge lamp comprising a sealed envelope containing a gaseous atmosphere and having a pair of electrodes sealed therein, means to confine the discharge between said electrodes to an eccentric path within said envelope close to one wall thereof to stabilize said discharge and to reduce the heating of the opposite wall of said envelope, and means to maintain that portion of the envelope to which the discharge is adjacent below the softening temperature, said means comprising a water cooled jacket in contact with that portion of said envelope.

VICTOR JAMES FRANCIS. JOHN WALTER RYDE. 

